1. Field of the Invention
This invention relates to a method for patterning an epitaxial substrate and a method for forming a light-emitting diode with a patterned epitaxial substrate.
2. Description of the Related Art
Referring to FIG. 1, a conventional light-emitting diode (LED) includes an epitaxial substrate 11, a light-emitting unit 12, and an electrode unit 13. When electricity is supplied to the light-emitting unit 12 through the electrode unit 13, the light-emitting unit 12 will emit light.
Using a III-nitride LED, e.g., a gallium nitride (GaN)-based LED as an example, the III-nitride LED is constructed by epitaxially growing GaN on a sapphire substrate 11. However, the large differences in the lattice constant and the coefficient of thermoexpansion (CTE) between GaN and the sapphire epitaxial substrate 11 result in a high density of threading dislocation during formation of the light emitting unit 12, thereby limiting the emitting efficiency thereof. In addition, since the light emitting unit 12 is made from semiconductor materials with high refractive index (GaN has a refractive index of 2.5), total internal reflection is likely to occur at an interface between the light emitting unit 12 and air (air has a refractive index of 1), thereby trapping the light in the light emitting unit 12. The light-trap reduces the light extraction efficiency and emitting efficiency of the LED.
In recent years, a patterned epitaxial substrate has been used to reduce the lattice dislocation and light-trap problem.
In J. J. Chen et al., IEEE Photonics Technology Letters, Vol. 20, pp. 1193-1195, 2008, it has been suggested that a sapphire substrate with nano-patterns has higher light extraction efficiency than that having micrometer magnitude patterns. In other words, under a same surface area of the sapphire substrate, reducing the geometrical size of the pattern can increase the density of the pattern on the sapphire, thus increasing output power and light extraction efficiency.
The main methods for constructing nano-patterns on an epitaxial substrate include: step and flash imprint lithography (M. Colburn et al., Proc. SPIE, Vol. 3676, pp. 379-389, 1999); using SiO2 nanospheres as etch mask to create a patterned sapphire substrate (B. Kim et al., Thin Solid Films, Vol. 516, pp. 7744-7747, 2008); contact-transferred and mask-embedded lithography (CMEL) (H. M. Lo et al., Superlattices and Microstructures, Vol. 48, pp. 358-364, 2010); and use of self-assembled metal (Ni) as an etch mask (H. Gao et al., Physica Status Solids, Vol. 205, pp. 1719-1723, 2008).
However, some drawbacks of these techniques exist. Step and flash lithography requires use of reactive ion etching and the apparatus for step and flash lithography is costly. CMEL and self-assembled metal (SAM) require metal vapor deposition, thus increasing manufacturing costs. Uneven nano-patterning in CMEL is also a problem that remains to be resolved. The cost for SiO2 nanospheres is high, and controlling the size of the SiO2 nanospheres is relatively difficult.
Accordingly, a simpler method for patterning the epitaxial substrate of the LED with the objective of obtaining higher light extraction efficiency is a main goal that is pursued in the field.